In her final presentation as U.S. poet laureate, Natasha Trethewey delivered a complex and searching lecture about the ideological evolution of Robert Penn Warren.

At the Library of Congress on Wednesday night, Trethewey began, as she often does, with her personal history and then moved into a rich exploration of America’s racial heritage.

As the child of an illegal mixed-race marriage in Mississippi, born on Confederate Memorial Day, Trethewey grew up in the shadow of Jim Crow, in what she called “the most Southern place on earth.” She remembers seeing a cross burning near her grandmother’s house. It would be easy for her to hold Warren’s early racist attitudes against him. But, instead, in her lecture — titled “‘The World of Action and Liability:’ on Saying What Happens” — she regarded him in a gracious and intellectually complicated way, charting his progress from apologist to enlightened citizen.

Though now best remembered for his Pulitzer Prize-winning novel, “All the King’s Men,” Warren also won two Pulitzers and a National Book Award for his poetry. And as the first U.S. poet laureate (1986-7), he helped define the position that Trethewey has held since 2012.

Born in Kentucky in 1905, Warren inherited the Confederate culture and defended segregation early in his life. For Trethewey, Warren’s “revisions and repudiations” of his racist ideals serve as “a blueprint for change … a mirror of our own reckoning.”

Reading quickly in a clear and resonant voice, Trethewey focused extensively on Warren’s dramatic poem, “Brother to Dragon.” Published in 1953, the poem concerns the murder of a slave by two relatives of Thomas Jefferson in 1811. Jefferson and R.P.W. (standing in for the author) are among several characters who reflect on the crime and the subsequent trial.

At times, the language of the poem is ugly and seems to blame the victim, but Trethewey claims that “this is only one position along the way. We see [Warren] grappling with difficult and contradictory ideals. We see his willingness to show an unattractive side to the self.”

Even when he reissued the poem in 1979, he resisted the temptation to “present himself in a tidy light,” she said, praising his honesty and his respect for history, no matter how unpleasant. “Warren made the choice to be consciously historical,” to fight “against amnesia and nostalgia.” Whether he ultimately arrived at modern-day enlightenment on the issue of race, “matters less,” she said, “than the journey he undertook.”

“We are all Mississippians,” she concluded. “We do what we must do to perfect our union.”

Like her poetry, it was an evening rich in the complexities of America’s past. After the talk, Trethewey signed copies her books, including the Pulitzer Prize-winning “Native Guard” (2006), which describes an all-black Louisiana regiment in the Union Army. Her white father, the poet Eric Trethewey, sat quietly beside her in the Great Hall of the library named after Thomas Jefferson, a slave owner who declared that “all men are created equal.”

Wednesday evening’s lecture marked the conclusion of Trethewey’s second term as poet laureate. Now 48, she is among the youngest people ever to hold the office and has clearly been one of the most popular. Librarian of Congress James Billington praised Trethewey for the way she has “consistently and dramatically expanded the power of this position.” In addition to holding office hours with the public, she traveled the country, and she reached millions through her innovative segment on the PBS NewsHour, “Where Poetry Lives.” Billington quipped, “Wherever she went, it lived.”

The Library of Congress is expected to name the next poet laureate in June.